A thermistor having positive temperature coefficient of resistance is known as a PTC (Positive Temperature Coefficient) thermistor. The resistance in the PTC thermistor increases as the temperature rises, so the PTC thermistor is used as a self-controlling heater, an over-current protection element, a temperature sensor or the like. In the past, the PTC thermistor was formed by adding a trace of rare earth based elements or the like into the main component of barium titanate (BaTiO3) to turn it into a semiconductor. The resistance of the thermistor is low under a temperature below the Curie temperature; however, it will be sharply increased by several orders of magnitude under a temperature above the Curie temperature.
Usually, the Curie temperature of barium titanate is about 120° C. When part of Ba is replaced by Sr or Sn, it will shift to a lower temperature. However, for shifting the Curie temperature towards a higher temperature, the current method is replacing part of Ba by Pb. Thus, an alternative material without using Pb is required to be applied from the viewpoint of the worldwide trend of reducing the environmental burden.
A method for preparing a PTC thermistor has been disclosed in the following Patent Document 1. In particular, more than one selected from Nb, Ta or rare earth elements is added to a composition with a structure of Ba1-2x(BiNa)xTiO3 ((0<X≦0.15) which is formed by replacing part of Ba with (BiNa) rather than Pb. The resultant mixture is sintered in a nitrogen atmosphere and then subjected to a thermal treatment in an oxidative atmosphere.
In addition, a Pb-free PTC thermistor has been disclosed in the following Patent Document 2 which is obtained by sintering a composition in a nitrogen atmosphere. In the mentioned composition, BamTiO3 based composition is used as the main component and the molar ratio m of A site to B site is set to be 1.001≦m≦1.01. In addition, part of Ba constituting the A site is replaced by Na, Bi, Ca and the rare earth elements, and Ca is contained with its molar ratio to 1 mol of the A site being 0.05 to 0.20.
Further, the following Patent Document 3 has disclosed a ceramic composition for PTC thermistor without using Pb. In this ceramic composition, Ba, Ti, Bi, O are all contained as the necessary elements, and one or more kinds of monovalent alkali metals are contained. The molar ratio X2 of the monovalent alkali metal to Bi is 0.65≦X2≦1.59, and 480 ppm or less of Nb is contained. In addition, the amount of Sb, Ta or each rare earth element is 10 ppm or less respectively.
It has been described in any of these Patent Documents that a PTC thermistor as follows could be provided. The PTC thermistor is Pb free and has its Curie temperature shifted to a temperature higher than 120° C. In addition, its resistivity at room temperature is low. Further, the ratio of the resistivity at room temperature and the increased resistivity above the Curie temperature (referred to as “PTC jump” hereinafter for convenience) are quite great.